Escaping from HTML

Everything outside of a pair of opening and closing tags is ignored by the
PHP parser which allows PHP files to have mixed content. This allows PHP
to be embedded in HTML documents, for example to create templates.

<p>This is going to be ignored by PHP and displayed by the browser.</p><?php echo 'While this is going to be parsed.'; ?><p>This will also be ignored by PHP and displayed by the browser.</p>

This works as expected, because when the PHP interpreter hits the ?> closing
tags, it simply starts outputting whatever it finds (except for an
immediately following newline - see
instruction separation)
until it hits another opening tag unless in the middle of a conditional
statement in which case the interpreter will determine the outcome of the
conditional before making a decision of what to skip over.
See the next example.

Using structures with conditions

Example #1 Advanced escaping using conditions

<?php if ($expression == true): ?> This will show if the expression is true.<?php else: ?> Otherwise this will show.<?php endif; ?>

In this example PHP will skip the blocks where the condition is not met, even
though they are outside of the PHP open/close tags; PHP skips them according
to the condition since the PHP interpreter will jump over blocks contained
within a condition that is not met.

For outputting large blocks of text, dropping out of PHP parsing mode is
generally more efficient than sending all of the text through
echo or print.

In PHP 5, there are up to five different pairs of opening and closing tags
available in PHP, depending on how PHP is configured. Two of these,
<?php ?> and
<script language="php"> </script>, are always
available. There is also the short echo tag <?= ?>,
which is always available in PHP 5.4.0 and later.

The other two are short tags and ASP style
tags. As such, while some people find short tags and
ASP style tags convenient, they are less
portable, and generally not recommended.

Note:

Also note that if you are embedding PHP within XML or XHTML
you will need to use the <?php ?> tags to remain
compliant with standards.

PHP 7 removes support for ASP tags and
<script language="php"> tags. As such, we recommend
only using <?php ?> and <?= ?> when
writing PHP code to maximise compatibility.

Example #2 PHP Opening and Closing Tags

1. <?php echo 'if you want to serve PHP code in XHTML or XML documents, use these tags'; ?>

2. You can use the short echo tag to <?= 'print this string' ?>. It's always enabled in PHP 5.4.0 and later, and is equivalent to<?php echo 'print this string' ?>.

3. <? echo 'this code is within short tags, but will only work '.'if short_open_tag is enabled'; ?>

5. <% echo 'You may optionally use ASP-style tags'; %> Code within these tags <%= $variable; %> is a shortcut for this code <% echo $variable; %> Both of these syntaxes are removed in PHP 7.0.0.

Short tags (example three) are only available when they are
enabled via the short_open_tagphp.ini configuration file directive, or if PHP was configured
with the --enable-short-tags
option.

ASP style tags (example five) are only
available when they are enabled via the
asp_tagsphp.ini configuration file
directive, and have been removed in PHP 7.0.0.

Note:

Using short tags should be avoided when developing applications
or libraries that are meant for redistribution, or deployment on
PHP servers which are not under your control, because short tags
may not be supported on the target server. For portable,
redistributable code, be sure not to use short tags.

Note:

In PHP 5.2 and earlier, the parser does not allow the
<?php opening tag to be the only thing in a file.
This is allowed as of PHP 5.3 provided there are one or more whitespace
characters after the opening tag.

Note:

Starting with PHP 5.4, short echo tag <?= is always recognized and
valid, regardless of the short_open_tag setting.

User Contributed Notes 8 notes

When the documentation says that the PHP parser ignores everything outside the <?php ... ?> tags, it means literally EVERYTHING. Including things you normally wouldn't consider "valid", such as the following:

<html><body><p<?php if ($highlight): ?> class="highlight"<?php endif;?>>This is a paragraph.</p></body></html>

Notice how the PHP code is embedded in the middle of an HTML opening tag. The PHP parser doesn't care that it's in the middle of an opening tag, and doesn't require that it be closed. It also doesn't care that after the closing ?> tag is the end of the HTML opening tag. So, if $highlight is true, then the output will be:

<html><body><p class="highlight">This is a paragraph.</p></body></html>

Otherwise, it will be:

<html><body><p>This is a paragraph.</p></body></html>

Using this method, you can have HTML tags with optional attributes, depending on some PHP condition. Extremely flexible and useful!

One aspect of PHP that you need to be careful of, is that ?> will drop you out of PHP code and into HTML even if it appears inside a // comment. (This does not apply to /* */ comments.) This can lead to unexpected results. For example, take this line:

<?php $file_contents = '<?php die(); ?>' . "\n";?>

If you try to remove it by turning it into a comment, you get this:

<?php// $file_contents = '<?php die(); ?>' . "\n";?>

Which results in ' . "\n"; (and whatever is in the lines following it) to be output to your HTML page.

The cure is to either comment it out using /* */ tags, or re-write the line as:

These methods are just messy. Short-opening tags and ASP-styled tags are not always enabled on servers. The <script language="php"></script> alternative is just out there. You should just use the traditional tag opening:

Lead to something along the lines of messy code. Writing your application like this can just prove to be more of an inconvenience when it comes to maintenance.

If you have to deal chunks of HTML, then consider having a templating system do the job for you. It is a poor idea to rely on the coding islands method as a template system in any way, and for reasons listed above.

" There are four different pairs of opening and closing tags which can be used in PHP. Two of those, <?php ?> and <script language="php"> </script>, are always available " - It is not true with PHP7. script and ASP style tags are removed from PHP7.